Priority level of the diagnostic used to control which diagnostics should be shown and filtered.
Typically this only makes sense to set to a different value than DiagnosticLevel.info for diagnostics representing properties. Some subclasses have a level argument to their constructor which influences the value returned here but other factors also influence it. For example, whether an exception is thrown computing a property value DiagnosticLevel.error is returned.
@override final DiagnosticLevel level;